The association of fibronectin with cultured cells and their matrices was studied by using models that included cultures of normal and malignant cells, isolated extracellular matrices of cultured human fibroblasts and purified protein components (actin, collagen, fibronectin). Specifically, (1) a 10,000 molecular weight peptide that causes the release of fibronectin from untransformed cells in culture was isolated, purified and characterized as well as 66,000 molecular weight protein that was particularly sensitive to this fibronectin releasing peptide; (2) antibodies against this 66,000 molecular weight protein were produced; (3) the binding-site for actin in fibronectin molecules was isolated; (4) fibronectin-releasing 10,000 molecular weight peptides with respect to biological activities were identified. The production of these factors by different normal and malignant cells was studied.